The term split-cycle engine as used in the present application may not have yet received a fixed meaning commonly known to those skilled in the engine art. Accordingly, for purposes of clarity, the following definition is offered for the term split-cycle engine as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a power piston slidably received within a power cylinder and operatively connected to the crankshaft such that the power piston reciprocates through a power (or expansion) stroke and an exhaust stroke during a single rotation of the crankshaft;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft; and
a gas passage interconnecting the power and compression cylinders, the gas passage including an inlet valve and an outlet (or crossover) valve defining a pressure chamber therebetween.
U.S. Pat. Nos. 6,543,225, 6,609,371, and 6,952,923, all assigned to the assignee of the present invention, disclose examples of split-cycle internal combustion engines as herein defined. These patents contain an extensive list of United States and foreign patents and publications cited as background in the allowance of these patents. The term “split-cycle” has been used for these engines because they literally split the four strokes of a conventional pressure/volume Otto cycle (i.e., intake, compression, power and exhaust) over two dedicated cylinders: one cylinder dedicated to the high pressure compression stroke, and the other cylinder dedicated to the high pressure power stroke.
Conventionally, in a split-cycle type engine, the inlet valve to the gas passage may be a check valve and the outlet valve may be a poppet valve. The inlet check valve permits the one way flow of compressed gas out of the compression cylinder while not allowing its return. Further, split-cycle type engines typically include an inlet valve controlling the flow of gas into the compression cylinder and an exhaust valve controlling flow of exhaust out of the power cylinder, both of which may also be poppet-type valves.
The compression stroke of a split-cycle engine discharges into a high pressure crossover passage, rather than to atmosphere. Accordingly, there is always some gas trapped between the compression piston and the compression cylinder head at top dead center that does not discharge into the crossover passage. This trapped gas must be re-expanded down to approximately one atmosphere before a charge of ambient air can enter the compression cylinder on the intake stroke.
As a result, the swept volume of the compression cylinder in a split-cycle engine must be made larger than that of a conventional engine for the same amount of intake charge. This added volume decreases volumetric efficiency and increases friction of the split-cycle engine. Accordingly, it is desirable to reduce the piston to head clearance of the compression cylinder in a split-cycle engine as much as possible.